Methods and devices for monitoring threads or yarns are well known, they using various technologies, for example capacitive or optical piezoelectric sensors.
Piezoelectric sensor devices, for example that described in EP 0117571, use for example a piezoelectric ceramic combined with a textile ceramic or other element for contact with the thread or yarn, to transmit vibrations, caused by the roughness of the yarn sliding on said element, to the piezoelectric ceramic. The main advantage of this technology is its almost absolute insensitivity to the presence of dirt or yarn residues in contact with the textile ceramic, the yarn itself automatically cleaning the point of contact with the ceramic.
However, this technology presents the main drawback of the absolute need for physical contact between the thread or yarn and the textile ceramic with which the piezoelectric ceramic is combined; this prevents monitoring of yarns which, for example, cannot be touched during their monitoring, to prevent undesired friction or rubbing, which could tear the thread or modify a controlled characteristic, such as tension.
Another drawback is the sensitivity of the piezoelectric ceramic to the presence of very intense vibrations or noises which by simulating the condition of the thread in its sliding state would prevent the device, in case of yarn breakage, from generating the signal for halting the textile machine.
In contrast, capacitive sensor devices measure the capacitance variation of said sensors caused by the variation in their dielectric, including the actual yarn subjected to monitoring. By measuring the variation in said dielectric, the state of movement the thread, its possible count variation, etc. can be measured. The advantage of this technology is the ability to monitor a yarn even without direct contact with it, the presence of dirt or yarn residues only slightly influencing the measurement capacity of said sensors.
The main drawback of these devices is however the impossibility of monitoring conductive threads or yarns, such as copper or steel threads or those in which carbon fibres are present; in the same manner, moist yarns or those wetted with water or treated with antistatic or conductive oils cannot be monitored. This is because a conductive thread or yarn or the presence of water would short-circuit the dielectric, hence making it impossible to measure and monitor the yarn.
Moreover the capacitive means present high sensitivity to electrical fields or electrostatic discharges, the presence of which could be mistaken for a moving thread condition, which often prevents its correct operation.
In contrast, optical sensor devices, see for example that described in EP 0519281, normally use a light transmitter and a receiver, between which the thread or yarn to be monitored is positioned.
The main advantage of this technology is the ability to monitor any type of thread or yarn, whether conductive or not, without any contact with it. The disadvantage of the technology is mainly its sensitivity to the formation of dirt and residues left by the monitored yarn on the receiver and/or transmitter.